Therapeutic products can be administered to a human or animal in a variety of ways and the administration method is often matched to the specific requirements of the therapeutic product and its intended action. While oral administration is typically preferred, some therapeutic products, such as insulin, have to be administered in such a way as to avoid the digestive system, or it may be beneficial to deliver them directly to the site of intended action.
The administration of therapeutic products to avoid the digestive tract is known as parenteral delivery and is typically achieved by administering the therapeutic product as a liquid formulation directly into the circulation. This is commonly performed using a syringe or equivalent device to deliver a bolus of therapeutic product, or an infusion system capable of continuous, and in some cases programmed, delivery of therapeutic product. It is clear that the controlled administration of the therapeutic product more adequately matches the clinical requirements of these products, often offering better therapeutic control and reducing toxicity.
There is a growing demand for intensive insulin therapies for controlling glucose in people with diabetes. These therapies require that the patient administer regular insulin in an attempt to mimic the daily pattern of insulin release in an individual without diabetes. The pattern of insulin release in people without diabetes is complex. Generally, there is a background level of insulin that acts to control a fasting glucose and this is supplemented by temporary increases that counteract glucose released from ingested meals.
To meet this demand a number of infusion systems have appeared based on positive pressure reservoirs working in cooperation with a pulsatile pumping chamber having one-way check-valves operating at the inlet and/or the outlet of the pumping chamber.
An exemplary prior art infusion system is described in U.S. Pat. No. 4,714,462. This document describes an infusion system where therapeutic product is held in a reservoir at a positive pressure just below the infusion pressure required to introduce the therapeutic product into an animal or human. Therapeutic product is withdrawn from the reservoir via a one-way check-valve into a chamber by drawing a bellows member under the action of a solenoid thus increasing the volume of the chamber. Upon release of the solenoid the bellows member under the action of a return spring forces the therapeutic product via an outlet fluid restrictor to an infusion site. The outlet restrictor functions as part of an infusion rate sensor. The system can be programmed by a user to provide both basal and bolus dosages of therapeutic product, such as insulin.
Many infusion systems are dedicated for use in managed care environments, such as hospitals and medical care facilities due to their complexity, and the restrictions they place on the patient's freedom of movement. In such large systems, a variety of mechanisms may be employed to drive the pumping chamber. Typically, an actuator is connected to the pumping chamber such that movement of the actuator displaces a member or diaphragm of the pumping chamber to pump the liquid therapeutic product.
The actuator described in U.S. Pat. No. 4,714,462 is a solenoid actuator comprising an armature which is a component part of a solenoid. The armature is biassed via a spring in a direction to reduce the volume of the pumping chamber. The solenoid is driven by an electronics module. When the solenoid is energised the armature is driven in a direction such that the volume of the pumping chamber is increased, which draws fluid from the positive pressure reservoir until the pumping chamber is full. The solenoid is subsequently de-energised and the actuator spring provides a bias force on the armature that drives to decrease the volume of the pumping chamber thus pumping fluid toward the infusion site.
Other known actuators use piezo-electric effects to drive the actuator. However, the solenoid and piezo-electric actuators have a problem in that, as they are reduced in size for use in micro-fluidic systems, the driving force achievable by these actuators becomes substantially reduced.
An example of a micro-pump is described in U.S. Pat. No. 4,152,098. The pump has a flexible, resilient diaphragm which operates as a moveable resilient wall of a pumping chamber, under electro-magnetic actuation of a plunger, to cause the pumping action. The pump comprises an armature or plunger made of magnetic steel slidably moveable in an anti-friction sleeve. An electro-magnetic coil arrangement surrounds the sleeve and the plunger is moveable up and down with respect to the sleeve on energising the coil arrangement. Whilst a fairly high pumping force is achieved for its size, the micro-pump, overall occupying a volume of around 4 cm3, is still too large for today's requirements.
In order to reduce the size of micro-pumps still further, whilst retaining sufficient driving force in the actuator to perform efficient, leak-free, reliable pumping, an improved actuator is required.
US2002/0037221A describes a wax micro-actuator for use in a micro-fluidic system. The micro-pump comprises a substrate having a heater member. The substrate and heater member form a first portion. A second portion is provided adjacent the first portion. The second portion includes a high actuating power polymer (HAPP) portion, at least one resin layer and a shield member. The second portion is selectively shaped to form a thermal expansion portion. A diaphragm member encapsulates the thermal expansion portion so that when power is applied to the heater portion, the HAPP portion expands against the diaphragm member causing it to deflect. As the temperature in the HAPP portion reaches its solid-liquid-transition-temperature, the specific volume of polymer increases. With further heat input from the heater layer, the HAPP portion undergoes a phase transition. During the phase transition, the specific volume increases dramatically causing deflection of the diaphragm member. The diaphragm member may be connected to, or may itself form part of a pumping chamber which increases and decreases in volume as the diaphragm deflects.
Various heating systems are known in the art for wax actuators but these predominately relate to relatively large actuator devices rather than micro-actuators. For example GB1204836 describes a wax actuator with an embedded heater filament. FR2731475 describes a wax actuator having a conductive heating element disposed external to the working cavity. This provides for inefficient heating of the wax.
The device of US2002/0037221A is essentially planar having a thin film, low power, heating element. This poses a number of problems. The heater element arrangement is such that the wax is heated inefficiently. Also, the volume of wax is limited as a result of poor heat distribution due to this arrangement.
There is therefore a need in the art for an improved micro-actuator having an expandable working medium, which is thermally efficient, thus enabling further size reductions, that is accurately controllable, and is scalable. These and other objects will become apparent from the following description of the invention.